Image alignment and similarity transform

I'm trying to align two images using maketform to get an affine transform but I am having trouble using the similarity matrix:

1] get 2 correspondence points from each image.

    A = [x1 y1; x2 y2];
    B = [x3 y3; x4 y4];

2] find the difference between each points for the similarity matrix:

    S = B/A

3] get the affine transform using maketform:

trans = maketform('affine', inv(S)');

Further down the road I plan to use the affine transform to align one image to the other.

However, before that I get an error at stage 3:

The final column of A must consist of zeroes, except for a one in the last row.

I'm not sure I'm using the right format for the similarity matrix S. Any help for this would be much appreciated.